Drawn steel wire

ABSTRACT

A drawn steel wire has a predetermined chemical composition; in a region of the drawn steel wire that is closer to an axis line than a depth of 100 μm from a surface of the drawn steel wire in a lengthwise-section that includes the axis line of the drawn steel wire, a metallographic structure includes 90% or more of a drawn pearlite by an area ratio; in a region of the drawn steel wire that is the depth of 100 μm from the surface of the drawn steel wire in the lengthwise-section, the metallographic structure includes 70% or more of the drawn pearlite by the area ratio; when D in units of millimeters represents a diameter of the drawn steel wire, σ HV  represents a standard deviation of a Vickers hardness of the surface of the drawn steel wire, and Rp 0.2  represents a yield strength of the drawn steel wire, σ HV &lt;(−9500×ln(D)+30000) exp(−0.003×Rp 0.2 ) is satisfied, and a tensile strength TS of the drawn steel wire is 1770 MPa or higher.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a high strength drawn steel wire havinga tensile strength TS of 1770 MPa or more.

Priority is claimed on Japanese Patent Application No. 2016-139744,filed on Jul. 14, 2016, the content of which is incorporated herein byreference.

RELATED ART

A bare drawn steel wire obtained by wire drawing a high carbon steelwire rod, or a coated drawn steel wire obtained by wire drawing a wirerod and thereafter coating the wire rod with Zn plating or the like isused for various applications such as a drawn steel wire for a bridgecable, a drawn steel wire for prestressed concrete, and a drawn steelwire used for various drawn steel wire ropes. Such drawn steel wires arerequired to have, as important properties, excellent torsional property(number of turns depending on the wire diameter) specified, for example,in the JIS G 3521 (hard drawn steel wire) standard as well as tensilestrength.

However, in general, in a torsion test of a drawn steel wire,longitudinal cracks called delamination easily occur as the strength ofthe drawn steel wire is increased. That is, it becomes difficult tosatisfy excellent torsional property as the strength of the drawn steelwire increases.

Regarding the above-described problem, in Patent Document 1, a drawnsteel wire in which the delamination during twisting is suppressed isproposed as a drawn steel wire having excellent torsional property.Patent Document 1 discloses that the delamination is suppressed byadjusting the surface layer hardness in a transverse section of a drawnsteel wire depending on the wire diameter.

However, it is considered that delamination occurs from the weakestpoint in the longitudinal direction of the drawn steel wire. Therefore,it is difficult to reliably suppress the delamination merely bycontrolling the surface layer hardness of a specific transverse section.

Patent Document 2 discloses a hot dip galvanized drawn steel wire whichsatisfies torsional property while suppressing proeutectoid cementite bycontrolling the TS of the drawn steel wire depending on the Si content,the Al content, and the wire diameter. However, in Patent Document 2,only the tensile strength of the drawn steel wire is controlled by thebalance between the Si content and Al content, and variations in thestructure or mechanical properties of the drawn steel wire forsuppressing delamination are not adjusted. Therefore, in Patent Document2, high strength and the suppression of the delamination are notsubstantially compatible with each other.

In the related art, it is considered that torsional property is improvedby suppressing the delamination. However, the inventors have found thatthere are cases where the number of turns (number of turns) untilfracture is decreased even when delamination does not occur. Therefore,in consideration of the safety of a structure, not only the delaminationis not occurred, but also a number of turns is sufficient, as thetorsional property, is required of a drawn steel wire.

Patent Document 3 discloses that the mass ratio between Ti and N isspecified, and the half-width of the (110) plane of ferrite and residualstress on the surface of a drawn steel wire are controlled to cause theyield ratio YR (the ratio between the yield strength YS and the tensilestrength TS) to be 80% or less, thereby obtaining a drawn steel wirewith no delamination occurred.

However, in Patent Document 3, a number of turns is not examinedalthough the delamination is examined.

PRIOR ART DOCUMENT Patent Document

[Patent Document 1] Japanese Patent No. 3984393

[Patent Document 2] Japanese Patent No. 3036393

[Patent Document 3] Japanese Patent No. 4377715

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

The present invention has been made against a background of theabove-described circumstances. An object of the present invention is toprovide a drawn steel wire having excellent torsional property, in whichdelamination does not occur in a torsion test and a sufficient number ofturns is shown.

Means for Solving the Problem

The inventors focused on flow stress due to torsional deformation in thelongitudinal direction and the circumferential direction of a drawnsteel wire regarding the behavior during occurring delamination, andexamined the suppression of the delamination and the improvement in anumber of turns. As a result, it was found that the unevenness of strainon the outermost surface due to torsional deformation is decreased byreducing the unevenness of flow stress of the outermost layer regardingthe yield stress and the wire diameter of the entire drawn steel wire,resulting in the improvement in torsional property, and the presentinvention has been completed.

The present invention has been made on the basis of the above-describedknowledge, and the gist thereof is as follows.

(1) A drawn steel wire according to an aspect of the present inventionincludes, as a chemical composition, by mass %, C: 0.75% to 1.10%, Si:0.10% to 1.40%, Mn: 0.10% to 1.0%, Al: 0% to 0.10%, Ti: 0% to 0.10%, Cr:0% to 0.60%, V: 0% to 0.10%, Nb: 0% to 0.10%, Mo: 0% to 0.20%, W: 0% to0.50%, B: 0% to 0.0030%, N: limited to 0.0060% or less, P: limited to0.030% or less, S: limited to 0.030% or less, and a remainder includingFe and impurities; in a region of the drawn steel wire that is closer toan axis line than a depth of 100 μm from a surface of the drawn steelwire in a lengthwise-section that includes the axis line of the drawnsteel wire, a metallographic structure includes 90% or more of a drawnpearlite by an area ratio; in a region of the drawn steel wire that isthe depth of 100 μm from the surface of the drawn steel wire in thelengthwise-section, the metallographic structure includes 70% or more ofthe drawn pearlite by the area ratio; when D in units of millimetersrepresents a diameter of the drawn steel wire, σ_(HV) represents astandard deviation of a Vickers hardness of the surface of the drawnsteel wire, and Rp_(0.2) represents a yield strength of the drawn steelwire, Expression (a) is satisfied; and a tensile strength of the drawnsteel wire is 1770 MPa or higher.

σ_(HV)<(−9500×ln(D)+30000)×exp(−0.003×Rp _(0.2))   (a)

(2) In the drawn steel wire according to (1), the chemical compositionmay include, by mass %, at least one selected from the group consistingof Al: 0.001% to 0.10%, Ti: 0.001% to 0.10%, Cr: more than 0% and 0.60%or less, V: more than 0% and 0.10% or less, Nb: more than 0% and 0.10%or less, Mo: more than 0% and 0.20% or less, W: more than 0% and 0.50%or less, and B: more than 0% and 0.0030% or less.

In the drawn steel wire according to (1) or (2), a coating layerincluding one or more of Zn, Al, Cu, Sn, Mg, and Si on the surface ofthe drawn steel wire may be provided.

In the present invention, as a yield strength YS, 0.2% proof stress(Rp_(0.2)) is employed.

Effects of the Invention

According to the aspect of the present invention, it is possible toobtain drawn steel wire having good torsional property by appropriatelycontrolling the chemical composition and the metallographic structure ofthe drawn steel wire, and suppressing the hardness distribution of thesurface of the drawn steel wire to be in an appropriate range dependingon the yield strength and the wire diameter of the drawn steel wire.Such a drawn steel wire is used as a drawn steel wire used forapplications for various ropes such as a bridge cable, a drawn steelwire for prestressed concrete, and ACSR, and besides, it is useful as adrawn steel wire used for applications in which torsion (twisting) isprimarily applied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of the surface of a drawn steel wire afterhardness measurement is performed on the surface of the drawn steelwire.

FIG. 2 is a graph showing the relationship between a Guy threshold and ayield strength (Rp_(0.2)), and torsional property of a drawn steel wirehaving a wire diameter of 5.0 mm to 5.4 mm in examples regarding each ofpresent invention examples and comparative examples.

FIG. 3 is a schematic view showing a method of determining a number ofturns in a torsion test.

EMBODIMENTS OF THE INVENTION

Hereinafter, a drawn steel wire according to an embodiment of thepresent invention (a drawn steel wire according to this embodiment) willbe described in detail.

<Chemical Composition>

First, the reason for limiting the chemical composition (composition) inthe drawn steel wire according to this embodiment will be described.Hereinafter, all % used for each chemical composition means mass %.

[C: 0.75% to 1.10%]

C is an element which contributes to high-strengthening of the drawnsteel wire by increasing the cementite fraction and refining thelamellar spacing of pearlite. When the C content is less than 0.75%, itis difficult to form pearlite as the principal structure. Therefore, theC content is set to 0.75% or more. The C content is preferably 0.77% ormore, and more preferably 0.80% or more. On the other hand, when the Ccontent exceeds 1.10%, proeutectoid cementite precipitates in a wire rodwhich is the material of the drawn steel wire, and the ductility of thewire rod is deteriorated. In this case, it becomes difficult to performwire drawing when the drawn steel wire is produced from the wire rod,and the ductility of the drawn steel wire is also deteriorated.Therefore, the C content is set to 1.10% or less. The C content ispreferably 1.05% or less, and more preferably 1.00% or less.

[Si: 0.10% to 1.40%]

Si is a deoxidizing element and is an element for solid solutionstrengthening of ferrite. When the Si content is less than 0.10%,sufficient hardenability cannot be secured during heat treatment. In acase where the drawn steel wire is subjected to zinc plating, it isdifficult to control an alloy layer. Therefore, the Si content is set to0.10% or more. The Si content is preferably 0.12% or more, and morepreferably 0.15% or more. On the other hand, when the Si content isexcessive, decarburization during heating is promoted, and theperformance for the mechanical descaling is deteriorated. In addition, anon-pearlite structure is increased during patenting. Therefore, the Sicontent is set to 1.40% or less. The Si content is preferably 1.30% orless, and more preferably 1.25% or less.

[Mn: 0.10% to 1.0%]

Mn is a deoxidizing element and is an element which improves thehardenability of steel. When the Mn content is less than 0.10%,sufficient hardenability cannot be secured during the heat treatment.Therefore, the Mn content is set to 0.10% or more. The Mn content ispreferably 0.20% or more, more preferably 0.30% or more. On the otherhand, when the Mn content exceeds 1.0%, a pearlitic transformation isdelayed and it is difficult to obtain a desired microstructure.

Therefore, the Mn content is set to 1.0% or less. The Mn content ispreferably 0.90% or less, and more preferably 0.80% or less.

The drawn steel wire according to this embodiment has the essentialelements described above, and the remainder thereof basically includesFe and impurities. However, in addition to each elements describedabove, one or more selected from the group consisting of Al, Ti, Cr, V,Nb, Mo, W, and B may be included in the drawn steel wire within theranges described below. That is, the drawn steel wire includes theessential elements and may include one or more selected from the groupconsisting of Al, Ti, Cr, V, Nb, Mo, W, and B, and the remainder thereofis Fe and impurities. Al, Ti, Cr, V, Nb, Mo, W, and B are optionalelements, and do not need to be necessarily included in the drawn steelwire. Therefore, the lower limit thereof is 0%.

In addition, the impurities are elements incorporated from the rawmaterials such as ore or scrap when steel is industrially manufactured,or from various environments in a manufacturing process, and are allowedin a range in which the properties of the steel are not adverselyaffected.

[Al: 0% to 0.10%]

Al is an element effective as a deoxidizing element. In a case ofobtaining this effect, it is preferable to set the Al content to 0.001%or more. The Al content is more preferably 0.005% or more, and even morepreferably 0.010% or more. On the other hand, when the Al content isexcessive, coarse hard inclusions are formed. In this case, drawabilityis deteriorated, and stability in continuous casting is deteriorated.Therefore, even in a case of including Al, the Al content is set to0.10% or less. The Al content is preferably 0.080% or less, and morepreferably 0.070% or less.

[Ti: 0% to 0.10%]

Ti is an element which is effective as a deoxidizing element and has anaction of fixing N in steel and improving drawability. Furthermore, Tiis an element which precipitates as Ti(C, N), functions as pinningparticles, and contributes to the refinement of austenite grains. In acase of obtaining these effects, it is preferable to set the Ti contentto 0.001% or more. The Ti content is more preferably 0.005% or more, andeven more preferably 0.010% or more. On the other hand, when the Ticontent is excessive, coarse TiN is formed in a casting stage, anddrawability is deteriorated. Therefore, even in a case of including Ti,the Ti content is set to 0.10% or less. The Ti content is preferably0.03% or less, and more preferably 0.025% or less.

[Cr: 0% to 0.60%]

Cr is an element which improves hardenability. In addition, Cr is anelement which improves the strength of the drawn steel wire by refiningthe lamellar spacing of pearlite. In a case of obtaining these effects,it is preferable to set the Cr content to be more than 0%. The Crcontent is more preferably 0.05% or more. On the other hand, Cr is anelement which stabilizes cementite. Therefore, when the Cr content isexcessive, not only the time until the end of a pearlitic transformationis increased, but also proeutectoid cementite is easily formed. Inaddition, the performance for the mechanical descaling is deteriorated.Therefore, even in a case of including Cr, the Cr content is set to0.60% or less. The Cr content is preferably 0.50% or less, and morepreferably 0.40% or less.

[V: 0% to 0.10%]

V is an element which improves hardenability, and is an element whichcontributes to the refinement of austenite grains when precipitated ascarbonitrides in an austenite region and contributes to strengthening ofthe drawn steel wire when precipitated in a ferrite region. In a case ofobtaining these effects, it is preferable to set the V content to morethan 0%. The V content is more preferably 0.05% or more.

On the other hand, when the V content is excessive, the time until theend of the pearlitic transformation is increased, and not only itbecomes difficult to form a required metallographic structure, but alsothe torsional property of the drawn steel wire is deteriorated due toprecipitation strengthening of carbonitride. Therefore, even in a caseof including V, the V content is set to 0.10% or less. The V content ispreferably 0.085% or less, and more preferably 0.070% or less.

[Nb: 0% to 0.10%]

Nb is an element which improves hardenability and is an element whichcontributes to the refinement of austenite grain sizes by itscarbonitride acting as pinning particles. In a case of obtaining theseeffects, it is preferable to set the Nb content to more than 0%. The Nbcontent is more preferably 0.003% or more.

On the other hand, when the Nb content is excessive, the time until theend of pearlitic transformation is increased, so that it becomesdifficult to form a required metallographic structure. Therefore, evenin a case of including Nb, the Nb content is set to 0.10% or less. TheNb content is preferably 0.04% or less, and more preferably 0.03% orless.

[Mo: 0% to 0.20%]

Mo is an element which improves the hardenability of steel and is anelement which contributes to the refinement of austenite grain sizes bya solute drug. In a case of obtaining these effects, it is preferable toset the Mo content to more than 0%. The Mo content is more preferably0.03% or more.

On the other hand, when the Mo content is excessive, the time until theend of the pearlitic transformation is increased, so that it becomesdifficult to form a required metallographic structure. Therefore, evenin a case of including Mo, the Mo content is set to 0.20% or less. TheMo content is preferably 0.10% or less, and more preferably 0.07% orless.

[W: 0% to 0.50%]

W is an element which improves the hardenability of steel. In a case ofobtaining this effect, it is preferable to set the W content to morethan 0%. The W content is more preferably 0.06% or more.

On the other hand, when the W content is excessive, the time until theend of the pearlitic transformation is increased, so that it becomesdifficult to form a required metallographic structure. Therefore, evenin a case of including W, the W content is set to 0.50% or less. The Wcontent is preferably 0.20% or less, and more preferably 0.10% or less.

[B: 0% to 0.0030%]

B is an element which segregates at the grain boundary in a solidsolution state and suppresses the formation of ferrite, therebyimproving drawability. In addition, B is an element having an action fordecreasing the amount of solute N by precipitating as BN. In a case ofobtaining these effects, it is preferable to set the B content to morethan 0%. The B content is more preferably 0.0003% or more.

On the other hand, when the B content is excessive, carbides of M₂₃(C,B)₆ precipitate at the grain boundary, and the drawability isdeteriorated. Therefore, even in a case of including B, the B content isset to 0.0030% or less. The B content is preferably 0.0025% or less.

In the drawn steel wire according to this embodiment, N, P, and S amongthe impurities are particularly harmful, so that the amounts thereofneed to be limited.

[N: 0.0060% or Less]

N is an element which deteriorates the torsional property of the drawnsteel wire when present in a solid solution state in steel and thusdeteriorates the drawability due to strain aging during wire drawing.Therefore, N is an element to be reduced as much as possible. When the Ncontent exceeds 0.0060%, variation in the hardness of the surface of thedrawn steel wire is increased, and the range specified in thisembodiment cannot be satisfied. Therefore, the N content is limited to0.0060% or less. The N content is preferably 0.0040% or less. The Ncontent is preferably small. However, when the N content is controlledto less than 0.0010%, the costs in actual production is significantlyincreased and it influences for controlling other impurities. Therefore,in consideration of the actual production, the N content may be set to0.0010% or more.

[P: 0.030% or Less]

P is an element which contributes to solid solution strengthening offerrite. At the same time, however, P is also an element whichsignificantly reduces the ductility of steel. In particular, when the Pcontent exceeds 0.030%, the drawability is significantly decreasedduring wire drawing from a wire rod to the drawn steel wire with adeterioration in ductility. Therefore, the P content is limited to0.030% or less. The P content is preferably limited to 0.020% or less,and is more preferably limited to 0.012% or less.

The P content is preferably small. However, when the P content islimited to less than 0.003%, the cost is significantly increased.Therefore, in consideration of the actual production, the P content maybe set to 0.003% or more.

[S: 0.030% or Less]

S is an element which causes red shortness and is also an element whichdecreases the ductility of steel. When the S content exceeds 0.030%, thedecrease in ductility becomes significant. Therefore, the S content islimited to 0.030% or less. The S content is preferably limited to 0.020%or less, and is more preferably limited to 0.010% or less.

The S content is preferably small. However, when the S content islimited to less than 0.003%, the cost is significantly increased.Therefore, in consideration of the actual production, the S content maybe set to 0.003% or more.

<Metallographic Structure of Drawn Steel Wire>

In the drawn steel wire according to this embodiment, it is effective toadjust the chemical composition as described above and simultaneouslymake the metallographic structure an appropriate structure in order toimprove the torsional property.

The metallographic structure of the drawn steel wire according to thisembodiment primarily includes drawn pearlite which is stretched by wiredrawing pearlite having a lamellar structure of ferrite and cementite.Specifically, the drawn pearlite indicates pearlite in which the ratiobetween the maximum length in the axial direction of pearlite grains andthe maximum thickness in the direction perpendicular thereto (maximumlength in the axial direction/maximum thickness in the directionperpendicular to the axis), that is, the aspect ratio is 1.05 or more,in a section (lengthwise-section) in an axial direction including theaxis line of the drawn steel wire, that is, in an lengthwise-sectionalong the wire drawing direction. There may be cases where, ferrite,proeutectoid cementite, bainite, or martensite is present as anon-pearlite structure in addition to the drawn pearlite in themetallographic structure. However, as the fraction (area ratio) of thesestructures is increased, the torsional property is deteriorated.Therefore, the area ratio of the drawn pearlite in a region (internalregion) of the drawn steel wire that is closer to an axis line than adepth of 100 μm from the surface of the drawn steel wire in thelengthwise-section is set to 90% or more. The area ratio thereof is morepreferably set to 95% or more. The area ratio of the drawn pearlite maybe 100%.

On the other hand, in the surface layer portion of the drawn steel wire,decarburization occurs or the cooling rate becomes faster than thatinside the wire rod in a patenting process for the wire rod, so that thefraction of ferrite, proeutectoid cementite, bainite, or martensite asthe non-pearlite structure other than the drawn pearlite tends to behigher than that inside the drawn steel wire.

However, as the area ratio of these structures is increased, variationin the hardness of the drawn steel wire is increased, and the torsionalproperty is deteriorated. Therefore, as described above, 90% or more ofthe drawn pearlite is secured in the internal region of thelengthwise-section of the drawn steel wire and then the area ratio ofthe drawn pearlite in the metallographic structure in the surface layerregion of the drawn steel wire is set to 70% or more, and preferably 85%or more. In this embodiment, the surface layer region of the drawn steelwire means a region from the surface of the drawn steel wire to a depthof 100 μm. That is, in the lengthwise-section of the drawn steel wire,the region from the surface of the drawn steel wire to a depth of 100 μmis the surface layer region, and a region that is closer to the axisline (center side) than the surface layer region is the internal region.

The area ratio of the drawn pearlite of the surface layer region is anaverage area ratio of the drawn pearlite in the region of thelengthwise-section from the surface to a depth of 100 μm.

Specifically, the area ratio of the drawn pearlite in the internalregion or the surface layer region of the lengthwise-section is obtainedas follows.

At the surface layer region of the lengthwise-section (a position at adepth of 50 μm from the surface), ¼×D (a position at a ¼ depth of thediameter D of the drawn steel wire from the surface), and ½×D (aposition at a ½ depth of the diameter D of the drawn steel wire from thesurface), five visual fields are observed at a magnification of2,000-fold using an optical microscope, and the photographs of thestructures in the observed visual fields are taken. Image analysis isperformed by marking the non-pearlite structure of the taken photographand the area ratio of pearlite is measured. Here, a region composed ofonly ferrite and a structure in which cementite is coarsely scattered inferrite are determined as the non-pearlite structure. In addition,pearlite in which the ratio between the maximum length in the axialdirection of pearlite grains and the maximum thickness in the directionperpendicular thereto (maximum length in the axial direction/maximumthickness in the direction perpendicular to the axis), that is, theaspect ratio is 1.05 or more is determined as the drawn pearlite.

A value obtained by averaging the area ratios of the drawn pearliteobtained from the photograph of the structure in the surface layerregion (a position of 50 μm from the surface) is determined as the arearatio of the drawn pearlite in the surface layer region.

In addition, a value obtained by averaging the area ratios of the drawnpearlite obtained from the photographs of the structures at ¼×D and ½×Dis determined as the area ratio of the drawn pearlite in the internalregion of the lengthwise-section.

<Variation in Hardness of Surface of Drawn Steel Wire>

It is considered that the hardness of the surface of the drawn steelwire affects the flow stress during torsional deformation. That is, whenthe hardness of the surface of the drawn steel wire varies, strain to beapplied during applying torsional deformation is becomes uneven. It isconsidered that the unevenness may cause the delamination or thefracture at a small number of turns (decrease in number of turns). As aresult of experiments and investigations by the inventors, it was foundthat in a case where a standard deviation (σ_(HV)) is used as variationin the Vickers hardness HV of the surface of the drawn steel wire, whenσ_(HV) satisfies Expression (1) in response to the diameter (D [mm]) andthe yield strength (Rp_(0.2)) of the drawn steel wire, the delaminationand the decrease in the number of turns can be reliably suppressed whenthe torsional deformation is applied.

σ_(HV)<(−9500×ln(D)+30000)×exp(−0.003×Rp _(0.2))   (1)

Therefore, in the drawn steel wire according to this embodiment, thestandard deviation σ_(HV) of the Vickers hardness HV on the surface ofthe drawn steel wire was specified to satisfy Expression (1). Here, itis preferable that the standard deviation of the Vickers hardness of thesurface of the drawn steel wire is calculated from a hardnessdistribution obtained for an area of 500 mm² or more at a density of 1points/mm² or more.

Specifically, the standard deviation σ_(HV) of the Vickers hardness ofthe surface of the drawn steel wire can be obtained by the followingmethod.

That is, using a portable Rockwell hardness tester, an indenter isvertically pressed against the surface of the drawn steel wire under aload of 5 kgf, and the hardness is measured. At this time, indentationof 800 points or more is performed at intervals of 1 mm or less in thecircumferential direction and the longitudinal direction of the drawnsteel wire. The obtained hardness is converted into Vickers hardness,and the standard deviation (σ_(HV)) is obtained on the basis of theconverted value.

In this embodiment, when the hardness is in terms of Rockwell hardness,the resolution of the numerical values of the variation is low.Therefore, a value converted into Vickers hardness using a conversiontable is used.

Regarding zinc plating performed on the drawn steel wire, after agalvanized layer is peeled off by dipping the drawn steel wire inhydrochloric acid containing an inhibitor, the variation in the hardnessmay be measured in the above-described manner.

<Tensile Strength>

Delamination tends to occur in a high strength drawn steel wire having atensile strength TS of 1770 MPa or more. Therefore, in this embodiment,a high strength drawn steel wire having a tensile strength TS of 1770MPa or more is targeted. The upper limit of the tensile strength of thedrawn steel wire according to this embodiment is not particularlylimited. However, from the viewpoint of ease of production, the upperlimit of the tensile strength may be about 2450 MPa.

<Torsional Property of Drawn Steel Wire>

The drawn steel wire according to this embodiment aims for not occurringdelamination and a number of turns of 20 times or more as the torsionalproperty.

The torsional property of the drawn steel wire is obtained by conductinga torsion test in which both ends of the drawn steel wire are chuckedand one side thereof is rotated, and measuring the number of turns andthe torque. The distance between grips in the torsion test is set to100×D (D is the wire diameter [mm]), and the torsion speed is set to 20rpm.

As shown in FIG. 3, when longitudinal cracks called delamination isoccurred, the torque is decreased. Therefore, occurring or not occurringthe delamination can be determined by measuring the torque. In addition,the delamination can be confirmed from the form of the fracturedsurface.

In this embodiment, the number of turns until the delamination occurs,or in a case where fracture occurs without delamination, the number ofturns until the fracture is used as the number of turns.

The diameter (wire diameter) of the drawn steel wire according to thisembodiment is not particularly limited, and may be determined asappropriate according to the product application, standards, and thelike. A typical diameter is about 1.5 mm to 7.0 mm.

Furthermore, the drawn steel wire according to this embodiment may beobtained by coating the surface of a high carbon drawn steel wire havingthe chemical composition, metallographic structure, and surface hardnessdistribution as described above with one or more metals of Zn, Al, Cu,Sn, Mg, and Si. That is, the drawn steel wire may be a coated drawnsteel wire having a coating layer including one or more of Zn, Al, Cu,Sn, Mg, and Si on the surface of the drawn steel wire according to thisembodiment. The coating layer may also be a plating layer.

A drawn steel wire used for a bridge cable, a drawn steel wire forprestressed concrete, and the like is subjected to zinc plating on thesurface for use in many cases, and a drawn steel wire used for powerapplications such as aluminium conductors steel reinforced (ACSR) isused in a state in which the surface is coated with Al, Cu, or the likein many cases.

<Production Method>

In order to produce the drawn steel wire according to this embodiment, aproduction method including, for example, the following steps may beapplied using steel that satisfies the above described conditions of thechemical composition as a material.

As long as each condition of the chemical composition or themetallographic structure of the drawn steel wire, and variation in thehardness of the surface of the drawn steel wire is in a range specifiedas above, an effect can be obtained regardless of the production method.Therefore, in a case where a drawn steel wire in which each condition ofthe chemical composition, metallographic structure, and variation in thehardness of the surface of the drawn steel wire is within the rangespecified as above is obtained by applying a process other than theprocess exemplified as follows, the drawn steel wire naturallycorresponds to the drawn steel wire according to this embodiment.

First, steel having the chemical composition as described above issubjected to casting and blooming by a known method, thereby producing asteel piece. Thereafter, the steel piece is heated to 1000° C. or higherand 1130° C. or lower. The heating temperature is preferably set to1000° C. or higher in order to complete austenitizing. In addition, theheating temperature is preferably 1130° C. or less, and more preferably1100° C. or less in order to suppress coarsening and duplex grainformation of austenite grains. In addition, the holding time after thepredetermined heating temperature is reached is preferably shorter than30 minutes in order to prevent promotion of decarburization of thesurface layer and to suppress duplex grain formation of austenitegrains.

A hot rolled steel is obtained by performing rough rolling and finishrolling on the steel piece after the heating. At this time, thetemperature of the finish rolling (finish temperature) is adjusted in arange of 850° C. to 980° C. When the finish rolling temperature is lowerthan 850° C., austenite grains are excessively refined and a pearlitictransformation becomes uneven. On the other hand, when the finishrolling temperature exceeds 980° C., it is difficult to control theaustenite grains in a subsequent cooling process. In addition, therolling reduction during the finish rolling is preferably 35% or more interms of cumulative rolling reduction in order to control the austenitegrains together with cooling process after winding process, which willbe described later.

The hot rolled steel after the hot rolling is held for 15 minutes orlonger at a temperature of not lower than 800° C., and the austenitegrains are adjusted by sufficiently recrystallizing the austenitegrains.

Next, the hot rolled steel after holding is directly dipped into amolten salt and is held at a temperature of 480° C. or higher and 580°C. or lower. Alternatively, the hot rolled steel is cooled to about roomtemperature by air blast cooling, thereafter heated to a temperature ofthe A3 point or higher (austenite region), and then dipped into moltenlead at 480° C. or higher and 600° C. or lower. The A3 point of thesteel can be obtained by a regression equation described in a knowndocument, for example, “Lectures, Modern Metallurgy, Materials Vol. 4,Ferrous Materials” p.43 and the like.

The hot rolled steel dipped into the molten salt or molten lead is wiredrawn to produce a drawn steel wire having a predetermined diameter. Inorder to control variation in the hardness of the surface layer of thedrawn steel wire during wire drawing, the final pass of the wire drawingat which the strength is maximized is important. Specifically, it iseffective to perform, as the final pass of the wire drawing, skin passwire drawing at a wire drawing rate of 5 m/min to 30 m/min, andpreferably 5 m/min to 25 m/min and at a reduction of area of 2.0% to10.0%.

When the wire drawing rate exceeds 30 m/min, heat generation due tofriction is increased, and thus the temperature of the drawn steel wireis increased. As a result, there is concern that σ_(HV) may beincreased. On the other hand, when the wire drawing rate is less than 5m/min, the amount of a lubricant pulled is decreased. When the amount ofthe lubricant pulled is decreased, there is concern that seizure mayoccur or the deformation heating amount may be increased, and thetemperature of the wire rod may be increased, resulting in an increasein σ_(HV).

Furthermore, when the reduction of area of the final pass (skin passwire drawing) exceeds 10.0%, the effect of suppressing variation in thehardness cannot be sufficiently obtained. On the other hand, when thereduction of area is less than 2.0%, it is difficult to uniformlyprocess the surface.

After the wire drawing, hot dip galvanizing or blueing, a heatstretching treatment, and the like may be performed as necessary.

EXAMPLES

Next, examples of the present invention will be described. Theconditions shown in the examples are merely examples adopted forconfirming the feasibility and effect of the present invention, and thepresent invention is not limited to these conditions. The presentinvention may adopt various conditions without departing from the gistof the present invention and as long as the object of the presentinvention is achieved.

Steel pieces having chemical compositions of kinds of steel A to T shownin Table 1 were subjected to heating, rolling, heat treatments, and wiredrawing under conditions shown in Table 2 to produce drawn steel wires.In the tables, DLP indicates direct patenting (direct in-line patenting)with molten salt after rolling, and LP indicates lead patenting. Holdingtime of Table 2 indicates a holding time at 800° C. or higher.

TABLE 1 Kind of Chemical composition (mass %) Remainder: Fe andimpurities steel C Si Mn P S Ti Al Cr V Nb Mo W N B A 0.76 0.25 0.770.010 0.006 0.010 — — — — — — 0.0031 — B 0.77 0.28 0.30 0.011 0.008 —0.052 — — — 0.06 — 0.0028 — C 0.82 0.21 0.30 0.007 0.005 — — — — — — —0.0043 — D 0.82 0.22 0.75 0.009 0.006 — 0.030 — — — — — 0.0039 — E 0.820.95 0.75 0.008 0.008 — 0.035 — — — — — 0.0048 — F 0.83 0.68 0.88 0.0100.005 — 0.035 — — 0.020 — — 0.0040 — G 0.83 0.15 0.95 0.007 0.006 —0.033 — 0.050 — — — 0.0038 — H 0.85 1.30 0.20 0.012 0.009 — 0.044 0.25 —— — 0.09 0.0022 — I 0.87 0.92 0.71 0.008 0.004 0.013 0.032 0.08 — — — —0.0038 — J 0.92 0.23 0.70 0.008 0.004 0.013 0.032 — — — — — 0.00380.0010 K 0.92 1.20 0.35 0.010 0.006 0.016 0.028 0.26 — — — — 0.0032 — L0.92 1.23 0.30 0.010 0.008 0.026 0.025 0.29 — — — — 0.0035 0.0010 M 0.970.24 0.69 0.006 0.004 0.008 0.028 0.05 — — — — 0.0031 — N 0.97 1.25 0.320.012 0.007 0.018 0.035 0.27 — — — — 0.0024 0.0015 O 0.99 0.90 0.350.010 0.005 — 0.035 0.10 0.060 — — — 0.0049 — P 1.02 0.65 0.75 0.0090.006 0.011 0.030 0.24 — 0.015 0.06 — 0.0030 0.0010 Q 1.09 1.10 0.400.010 0.008 0.010 — 0.05 — — — — 0.0040 — R 0.65 0.90 0.72 0.008 0.0020.008 — — — — — — 0.0035 — S 0.93 0.21 0.66 0.005 0.006 — 0.035 — — — —— 0.0100 — T 1.02 1.50 0.75 0.010 0.007 — 0.032 — — — — — 0.0020 —

TABLE 2 Production conditions Heat treatment Hot rolling Holding Coldworking Steel piece Cumulative time Re- Drawn Final pass heating finishFinish after heating Solvent steel Wire Kind Temper- rolling temper-Wire finish Heat temper- temper- wire drawing Reduction Post- of atureTime reduction ature diameter rolling treat- ature ature diameter rateof area treat- No. steel [° C.] [min] [%] [° C.] [mm] [sec] ment [° C.][° C.] [mm] [m/min] [%] ment  1 A 1100 20 35 900 13 20 DLP — 550 4.0 207.1 Blueing  2 A 1100 20 35 900 13 20 LP 900 580 4.0 20 7.1 Blueing  3 B1130 20 50 920 11 20 DLP — 550 3.2 20 8.8 Zinc plating  4 C 1080 15 35860 14 16 DLP — 520 5.0 20 7.5 Blueing  5 D 1130 10 60 920 5.5 16 DLP —550 1.8 28 8.3 —  6 E 1080 15 40 900 11 20 DLP — 550 5.2 20 7.3 Zincplating  7 E 1080 15 40 900 11 20 LP 900 575 5.2 20 7.3 Zinc plating  8F 1100 10 45 900 8 16 DLP — 550 2.8 20 6.8 Blueing  9 G 1130 25 60 900 816 DLP — 575 2.8 20 6.8 Zinc plating 10 H 1080 25 40 850 12 16 DLP — 5504.5 20 8.3 Zinc plating 11 I 1080 15 35 860 14 16 DLP — 550 5.4 20 7.0Zinc plating 12 J 1080 15 40 900 13 20 DLP — 550 5.0 20 7.5 — 13 K 108020 35 860 12 16 DLP — 550 5.0 20 7.5 Zinc plating 14 L 1100 20 35 900 1416 DLP — 550 7.0 20 8.1 Zinc plating 15 M 1130 25 45 900 9 16 DLP — 5503.0 20 9.3 Zinc plating 16 N 1100 15 35 900 14 16 DLP — 550 7.0 20 8.1Zinc plating 17 N 1100 15 35 900 14 16 DLP — 550 7.0 20 8.1 — 18 N 110015 35 900 14 16 DLP — 550 5.4 20 7.0 Zinc plating 19 N 1100 15 45 900 1220 DLP — 550 5.0 20 7.5 Blueing 20 O 1100 15 45 900 12 20 DLP — 550 5.020 7.5 — 21 P 1080 15 35 900 12 20 DLP — 550 5.2 20 5.5 Zinc plating 22Q 1080 15 35 900 16 20 DLP — 550 7.0 20 8.1 — x1 A 1100 20 35 900 13 20DLP — 550 4.0 120 7.1 Blueing x2 D 1130 10 60 920 5.5 16 DLP — 550 1.8120 8.3 — x3 F 1100 10 45 900 8 16 DLP — 550 2.8 20 16.3 Blueing x4 G1130 25 60 900 8 16 DLP — 575 2.8 120 6.8 Zinc plating x5 H 1080 25 40850 12 16 DLP — 550 4.5 120 8.3 Zinc plating x6 I 1080 60 35 860 14 16DLP — 550 5.4 20 5.6 Zinc plating x7 I 1160 25 35 860 14 16 DLP — 5505.4 20 5.6 Zinc plating x8 K 1080 20 35 860 12 16 DLP — 550 4.6 40 4.2Blueing x9 M 1130 25 45 900 9 16 DLP — 550 3.0 120 9.3 Zinc plating x10 N 1100 20 10 900 14 24 DLP — 550 7.0 20 8.1 Zinc plating x11  N 1130 1535 1000 14 16 DLP — 550 7.0 20 8.1 Zinc plating x12  N 1180 25 35 900 1416 DLP — 550 7.0 20 8.1 Zinc plating x13  N 1100 15 45 900 12 20 DLP —550 5.0 120 7.5 Blueing x14  N 1100 15 45 900 12 20 DLP — 550 5.0 2014.3 Blueing x15  N 1100 15 45 900 12 20 DLP — 550 5.0 120 14.3 Blueingx16  R 1080 20 35 900 16 20 DLP — 550 5.0 20 7.5 Blueing x17  S 1080 1540 900 14 16 DLP — 550 5.0 20 5.2 — x18  T 1130 15 35 900 14 16 DLP —550 5.4 20 5.6 Zinc plating x19  N 1220 120 35 900 14 16 DLP — 550 7.020 8.1 Zinc plating

On the obtained drawn steel wires, a tensile test, metallographicstructure observation, surface hardness measurement, and a torsion testwere conducted.

<Tensile Test>

According to the method described in JIS G 3521, the tensile test of thedrawn steel wire was conducted under conditions of a distance betweengrips of 200 mm, a distance between gauges of 50 mm, a tensile rate of10 mm/min, and the tensile strength TS and the yield strength YS (0.2%proof stress Rp_(0.2)) were measured.

<Metallographic Structure Observation>

At the surface layer region of the lengthwise-section (a position at adepth of 50 μm from the surface), ¼×D (a position at a ¼ depth of thediameter D of the drawn steel wire from the surface), and ½×D (aposition at a ½ depth of the diameter D of the drawn steel wire from thesurface), five visual fields were observed at a magnification of2,000-fold using an optical microscope, and the photographs of thestructures in the observed visual fields were taken. Image analysis wasperformed by marking the non-pearlite structure of the taken photographand the area ratio of drawn pearlite was measured. At this time, aregion composed of only ferrite and a structure in which cementite wascoarsely scattered in ferrite were determined as the non-pearlitestructure. In addition, pearlite in which the ratio between the maximumlength in the axial direction of pearlite grains and the maximumthickness in the direction perpendicular thereto (maximum length in theaxial direction/maximum thickness in the direction perpendicular to theaxis), that is, the aspect ratio was 1.05 or more was determined as thedrawn pearlite.

A value obtained by averaging the area ratios of the drawn pearlite ofeach of the visual fields obtained from the photograph of the structurein the surface layer region was determined as the area ratio of thedrawn pearlite in the surface layer region of the lengthwise-section.

In addition, a value obtained by averaging the area ratios of thepearlite obtained from the photographs of the structures at ¼×D and ½×Dwas determined as the area ratio of the drawn pearlite in the internalregion of the lengthwise-section.

<Surface Hardness Measurement>

The hardness of the surface of the drawn steel wire was measured by aportable Rockwell hardness tester. An indenter was vertically drivenwith a load of 5 kgf against the surface of the drawn steel wire, andthe hardness was measured. The hardness was obtained by performingindentation of 800 points or more at intervals of 1 mm or less in thecircumferential direction and the longitudinal direction of the drawnsteel wire. FIG. 1 shows an example of an external appearance photographof the drawn steel wire surface of the drawn steel wire driven by theindenter.

Each of hardnesses obtained was converted into a Vickers hardness, andthe standard deviation (σ_(HV)) was obtained from the converted value.

From the value of the yield strength obtained by the tensile test andthe wire diameter (diameter of the drawn steel wire), the threshold ofthe standard deviation corresponding to the right side of Expression (1)was obtained. Then, by comparing these values, variation in the hardnessof the surface of the drawn steel wire was evaluated.

In addition, for the drawn steel wire subjected to zinc plating, theplating layer was peeled off by dipping the drawn steel wire inhydrochloric acid containing an inhibitor, and the variation in thehardness was measured in the above-described manner.

<Torsion Test>

Evaluation of the torsional property of each of the drawn steel wireswas performed on the basis of the torsion test method of JIS G 3521 byconducting a torsion test in which both ends of the drawn steel wirewere chucked and one side thereof was rotated, and measuring the numberof turns and the torque. The form of the fractured surface was checked.In the torsion test, the distance between grips was set to 100×D (D isthe wire diameter [mm]), and the torsion speed was set to 20 rpm.

The number of turns until the delamination occurred, or in a case wherefracture occurred without delamination, the number of turns until thefracture was used as the number of turns. In a case where the number ofturns was 20 times or more without delamination, excellent torsionalproperty was determined.

Table 3 shows the properties of each of the drawn steel wires obtained.

TABLE 3 Properties of drawn steel wire Metallographic structure Arearatio of drawn Area ratio of drawn Surface hardness Torsional propertyTensile properties pearlite in surface pearlite in σ_(HV) Number of TSYS Yield layer region internal region σ_(HV) threshold turns No. [MPa][MPa] ratio [%] [%] [HV] [HV] (Times) Delamination Division 1 2200 18600.85 95 98 60 63.5 25 Not occurred Present 2 2180 1870 0.86 92 98 5561.6 30 Not occurred Invention 3 1980 1750 0.88 90 99 45 99.4 24 Notoccurred 4 2020 1720 0.85 93 96 55 84.5 29 Not occurred 5 2240 1690 0.7595 99 70 153.4 31 Not occurred 6 1830 1710 0.93 85 98 48 84.8 27 Notoccurred 7 1880 1730 0.92 80 99 56 79.9 28 Not occurred 8 2200 1880 0.8590 97 70 71.8 29 Not occurred 9 2060 1860 0.90 91 98 60 76.3 26 Notoccurred 10 2160 1900 0.88 78 95 48 52.6 21 Not occurred 11 2020 18700.93 82 97 42 51.2 24 Not occurred 12 2120 1650 0.78 95 99 85 104.2 28Not occurred 13 2100 1860 0.89 88 95 52 55.5 22 Not occurred 14 19601770 0.90 78 96 45 56.9 20 Not occurred 15 2120 1880 0.89 95 98 49 69.524 Not occurred 16 2060 1860 0.90 80 97 42 43.4 21 Not occurred 17 20901750 0.84 80 97 53 60.4 28 Not occurred 18 2210 1980 0.90 80 97 35 36.821 Not occurred 19 2250 1960 0.87 85 97 38 41.1 22 Not occurred 20 23201860 0.80 82 97 51 55.5 27 Not occurred 21 2150 1880 0.87 92 96 45 50.923 Not occurred 22 2220 1780 0.80 95 98 53 55.2 26 Not occurred x1  22301940 0.87 95 98 62 49.9 12 Occurred Comparative x2  2280 1850 0.81 95 99105 94.9 10 Occurred Example x3  2200 1880 0.85 90 97 78 71.8 14Occurred x4  2050 1900 0.90 91 98 85 76.3 8 Occurred x5  2140 1920 0.8878 95 67 52.6 5 Occurred x6  2010 1870 0.93 66 94 55 51.2 8 Occurred x7 2040 1900 0.93 69 97 58 46.8 5 Occurred x8  2250 2000 0.89 88 95 45 38.416 Not occurred x9  2110 1890 0.89 95 98 72 69.5 18 Occurred x10 20301880 0.93 82 96 58 40.9 4 Occurred x11 2060 1900 0.92 85 95 48 38.5 7Occurred x12 2010 1840 0.92 68 98 67 46.1 3 Occurred x13 2300 2040 0.8785 97 72 32.3 10 Occurred x14 2250 1990 0.87 85 97 40 37.6 15 Occurredx15 2280 2010 0.87 85 97 45 35.4 12 Occurred x16 2060 1850 0.90 65 88 6557.2 11 Occurred x17 2190 1750 0.80 95 97 88 77.2 13 Occurred x18 22402050 0.92 55 89 47 29.8 4 Occurred x19 2010 1830 0.91 30 97 45 47.5 15Occurred

Test Nos. 1 to 22 shown in Tables 1 to 3 are examples (present inventionexamples) of the drawn steel wires which satisfy each of the conditionsspecified in the present invention, and it was confirmed that all theexamples were excellent in torsional property.

On the other hand, in Test Nos. ×1 to ×19 of comparative examples,production conditions such as the chemical compositions or the wiredrawing conditions were not appropriate, and conditions for themetallographic structure and/or the variation in the surface hardnessdeviated from the ranges specified in the present invention. As aresult, good torsional property was not obtained.

FIG. 2 shows the relationship between the σ_(HV) threshold (the value onthe right side of (1) described above) and the yield strength, andtwisting properties of the drawn steel wires having a wire diameter in arange of 5.0 mm to 5.4 mm among the present invention examples and thecomparative examples in the examples. In FIG. 2, an O mark indicatesthat delamination had not occurred and the number of turns was 20 timesor more, and an X mark indicates that the number of turns was less than20 times. It is apparent from FIG. 2 that high strength and excellenttorsional property can be obtained within the ranges specified in thepresent invention.

While the preferred embodiments and examples of the present inventionhave been described above, these embodiments and examples are merelyexamples within the scope of the gist of the present invention, andadditions, omissions, substitutions, and other changes regarding theconfiguration are possible without departing from the gist of thepresent invention. That is, the present invention is not limited by theabove description but is limited only by the claims, and appropriatechanges can be made within the scope thereof.

1. A drawn steel wire comprising, as a chemical composition, by mass %:C: 0.75% to 1.10%, Si: 0.10% to 1.40%, Mn: 0.10% to 1.0%, Al: 0% to0.10%, Ti: 0% to 0.10%, Cr: 0% to 0.60%, V: 0% to 0.10%, Nb: 0% to0.10%, Mo: 0% to 0.20%, W: 0% to 0.50%, B: 0% to 0.0030%, N: limited to0.0060% or less, P: limited to 0.030% or less, S: limited to 0.030% orless, and a remainder including Fe and impurities; wherein in a regionof the drawn steel wire that is closer to an axis line than a depth of100 μm from a surface of the drawn steel wire in a lengthwise-sectionthat includes the axis line of the drawn steel wire, a metallographicstructure includes 90% or more of a drawn pearlite by an area ratio; ina region of the drawn steel wire that is the depth of 100 μm from thesurface of the drawn steel wire in the lengthwise-section, themetallographic structure includes 70% or more of the drawn pearlite bythe area ratio; when D in units of millimeters represents a diameter ofthe drawn steel wire, Gm/represents a standard deviation of a Vickershardness of the surface of the drawn steel wire, and Rp_(0.2) representsa yield strength of the drawn steel wire, Expression (1) is satisfied;and a tensile strength of the drawn steel wire is 1770 MPa or higher.σ_(HV)<(−9500×ln(D)+30000)×exp(−0.003×Rp _(0.2))   (1)
 2. The drawnsteel wire according to claim 1, wherein the chemical compositionincludes, by mass %, at least one selected from the group consisting ofAl: 0.001% to 0.10%, Ti: 0.001% to 0.10%, Cr: more than 0% and 0.60% orless, V: more than 0% and 0.10% or less, Nb: more than 0% and 0.10% orless, Mo: more than 0% and 0.20% or less, W: more than 0% and 0.50% orless, and B: more than 0% and 0.0030% or less.
 3. The drawn steel wireaccording to claim 1, wherein a coating layer including one or more ofZn, Al, Cu, Sn, Mg, and Si on the surface of the drawn steel wire isprovided.
 4. The drawn steel wire according to claim 2, wherein acoating layer including one or more of Zn, Al, Cu, Sn, Mg, and Si on thesurface of the drawn steel wire is provided.